Method of preparing dispersions of gelled polymeric microparticles and products produced thereby

ABSTRACT

Dispersions containing crosslinked acrylic polymer microparticles having a particle size of from 0.1 to 10 microns are produced in relatively high concentrations by a method comprising the free radical addition copolymerization of at least one monoethylenically unsaturated monomer with alpha, beta-ethylenically unsaturated monocarboxylic acid and crosslinking monomer selected from the group consisting of (1) epoxy group-containing compound and (2) a mixture of alkylenimine and organoalkoxysilane in the presence of a dispersion stabilizer and an aliphatic hydrocarbon dispersing liquid in which the crosslinked polymer particles are insoluble. 
     The crosslinked acrylic polymer microparticles are useful as additives to protective and decorative coatings.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of Application Ser. No.559,949, filed Mar. 19, 1975, now abandoned.

BACKGROUND OF THE INVENTION

Methods of preparing crosslinked acrylic polymer microparticles commonlyreferred to as microgel particles are known in the art. One such methodis disclosed in commonly-assigned copending application Ser. No.296,700, filed Oct. 11, 1972, now U.S. Pat. No. 3,880,796, in the nameof Roger M. Christenson et al. In this method, a non-aqueous polymerdispersion is prepared by polymerizing an ethylenically unsaturatedmonomer containing hydroxyl groups in the presence of (1) a dispersingliquid which is a solvent for the monomer but in which the resultantpolymer is insoluble and (2) a dispersion stabilizer. The resultantnon-aqueous polymer dispersion produced by this method consists of amajor proportion of uncrosslinked acrylic polymer particles and a minorproportion (e.g., 10 percent by weight or less) of crosslinked acrylicpolymer particles (i.e., microgel particles). Accordingly, in thismethod, it is necessary to separate the microgel particles from theuncrosslinked polymer particles. This is accomplished by the addition tothe dispersion of an active solvent for the uncrosslinked polymerparticles, thereby converting the dispersion to essentially a solution,but for the presence of the insoluble microgel particles. The microgelparticles are then separated from the bulk of the polymer byconventional means such as centrifuging, filtering, and the like.

The above process, while advantageous in some respects, has severalserious disadvantages. Thus, as will be apparent, the microgel particlesare an incidental by-product of the non-aqueous dispersion process andtherefore the yield is relatively low (e.g., 5 to 10 percent by weightor less). Moreover, because of this factor, it is necessary to separatethe microgel particles from a dispersion which contains a majorproportion of uncrosslinked acrylic polymer particles by dissolving theuncrosslinked polymer particles with an active solvent.

Still another method for producing microgel particles is disclosed inBritish Pat. No. 967,051 to Bullitt et al, dated Aug. 19, 1964. In thismethod, microgel particles are prepared by forming an aqueous emulsionof monoethylenic unsaturated monomer and a crosslinking monomercontaining at least two ethylenic double bonds, heating the emulsion toa temperature of about 40° to 100° C. until the reaction issubstantially complete to yield a microgel and during the reactionadding an agent to inhibit the formation of high molecular weightsubstantially uncrosslinked material. The inhibiting agent as disclosedin Bullitt et al can be an active solvent for the monomers or a chaintransfer agent. This method has several disadvantages. Thus, the methodutilizes conventional emulsion polymerization techniques requiringcareful control of the process to prevent settling and the like.Further, the use of crosslinking monomers containing at least 2ethylenic double bonds (e.g., divinyl and diacrylate monomers) has beenfound to lead to flocculation problems in relatively high solids level(i.e., 40 percent by weight or higher) microgel particle dispersions.Finally, this method requires the additional step of adding awater-immiscible solvent or chain transfer agent to the reactionmixture.

BRIEF SUMMARY OF THE INVENTION

The method of the present invention overcomes essentially all of thedisadvantages of the prior art. Thus, the present invention provides amethod of producing crosslinked acrylic polymer microparticles inrelatively high concentrations (i.e., solids levels of 20 to 60 percentby weight) by a process which comprises the free radical additioncopolymerization of alpha, beta-ethylenically unsaturated monocarboxylicacid, at least one other copolymerizable monoethylenically unsaturatedmonomer and crosslinking monomer selected from the group consisting of(1) epoxy group-containing compound and (2) a mixture of alkylenimineand organoalkoxysilane in the presence of a polymeric dispersionstabilizer and dispersing liquid in which the cross-linked acrylicpolymer particles are insoluble. The reaction is carried out at elevatedtemperatures such that the dispersion polymer first forms and then iscrosslinked; usually the temperature should be between about 50° C. and150° C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Examples of alpha, beta-ethylenically unsaturated monocarboxylic acidwhich may be used are acrylic acid, methacrylic acid, ethacrylic acid,alpha-chloroacrylic acid, crotonic acid isocrotonic acid, tiglic acidand angelic acid. The preferred alpha, beta-ethylenically unsaturatedmonocarboxylic acids are acrylic acid and methacrylic acid. Methacrylicacid is especially preferred. The amount of alpha, beta-ethylenicallyunsaturated monocarboxylic acid employed in the process of the inventionis usually in the range of from about 0.5 percent to about 15 percent byweight of the monomers used in the copolymerization process.

Various other monoethylenically unsaturated monomers may becopolymerized with the acid monomer in the process of this invention.Although essentially any copolymerizable monoethylenic monomer may beutilized, depending upon the properties desired, the preferredmonoethylenically-unsaturated monomers are the alkyl esters of acrylicor methacrylic acid, particularly those having from about 1 to about 4atoms in the alkyl group. Illustrative of such compounds are the alkylacrylates, such as methyl acrylate, ethyl acrylate, propyl acrylate, andbutyl acrylate and the alkyl methacrylates, such as methyl methacrylate,ethyl methacrylate, propyl methacrylate and butyl methacrylate. Otherethylenically unsaturated monomers which may advantageously be employedinclude, for example, the vinyl aromatic hydrocarbons, such as styrene,alpha-methyl styrene, vinyl toluene, unsaturated esters of organic andinorganic acids, such as vinyl acetate, vinyl chloride and the like, andthe unsaturated nitriles, such as acrylonitrile, methacrylonitrile,ethacrylonitrile, and the like. From about 70 percent to about 99percent by weight of such monoethylenically unsaturated monomers, basedon the weight of monomer solids can be utilized.

As indicated above, the crosslinking monomer employed in the process ofthe invention is selected from the group consisting of (1) epoxygroup-containing compound and (2) a mixture of alkylenimine andorganoalkoxysilane.

A particularly preferred class of epoxy group-containing compounds whichmay be utilized in the practice of the invention are monoepoxidecompounds which additionally contain ethylenic unsaturation.Illustrative of such preferred compounds are, for example, glycidylacrylate and glycidyl methacrylate.

Various alkylenimines can be utilized in the practice of the inventionincluding substituted alkylenimines. The preferred class of such aminesare those of the formula: ##STR1## where R₁, R₂, R₃, R₄ and R₅ are eachhydrogen; alkyl, such as methyl, ethyl, propyl, or the like, having, forexample, up to about 20 carbon atoms; aryl, such as phenyl or the like;aralkyl, such as tolyl, xylyl or the like; or aralkyl, such as benzyl,phenethyl or the like. R₆ in the above formula is hydrogen or a loweralkyl radical usually having not more than about 6 carbon atoms, and nis an integer from 0 to 1.

It is intended that the groups designated by the above formula includesubstituted radicals of the classes indicated where the substituentgroups do not adversely affect the basic nature of the imine in thereaction. Such substituents can include the groups such as cyano, halo,amino, hydroxy, alkoxy, carbalkoxy and nitrile. The substituted groupsmay thus be cyanoalkyl, haloalkyl, aminoalkyl, hydroxyalkyl,alkoxyalkyl, carbalkoxyalkyl, and similar substituted derivatives ofaryl, alkaryl and aralkyl groups where present.

A number of specific examples of alkylenimines within the classdescribed are as follows:

Ethylenimine (aziridine)

1,2-propylenimine (2-methyl aziridine)

1,3-propylenimine (azetidine)

1,2-dodecylenimine (2-decyl aziridine)

1,1-dimethyl ethylanimine (2,2-dimethyl aziridine)

Phenyl ethylenimine (2-phenyl aziridine)

Benzyl ethylenimine (2-phenylmethyl aziridine)

Hydroxyethyl ethylenimine (2-(2-hydroxyethyl)aziridine)

Aminoethyl ethylenimine (2-(2-aminoethyl)aziridine)

2-methyl propylenimine (2-methyl azetidine)

3-chloropropyl ethylenimine (2-(3-chloropropyl)aziridine)

Methoxyethyl ethylenimine (2-(2-methoxyethyl)aziridine)

Dodecyl aziridinyl formate (dodecyl 1-aziridinyl carboxylate)

N-ethyl ethylenimine (1-ethyl aziridine)

N-(2-aminoethyl)ethylenimine (1-(2-aminoethyl)aziridine)

N-(phenethyl)ethylenimine (1-(2-phenylethyl)aziridine)

N-(2-hydroxyethyl)ethylenimine (1-(2-hydroxyethyl)aziridine)

N-(cyanoethyl)ethylenimine (1-cyanoethyl aziridine)

N-phenyl ethylenimine (1-phenyl aziridine)

N-(p-chlorophenyl)ethylenimine (1-(4-chlorophenyl)aziridine)

Because of their availability and because they have been found to beamong the most effective, the preferred imines arehydroxyalkyl-substituted alkylenimines, such as N-hydroxyethylethylenimine and N-hydroxyethyl propylenimine.

Organoalkoxysilane monomers which may advantageously be employed in thepractice of this invention are the acrylatoalkoxysilanes,methacrylatoalkoxysilanes and the vinylalkoxysilanes. Illustrative ofsuch compounds are acryloxypropyltrimethoxysilane,gamma-methacryloxypropyltrimethoxysilane,gamma-methacryloxypropyltriethoxysilane,gamma-methacryloxypropyl-tris(2-methoxyethoxy)silane,vinyltrimethoxysilane, vinyltriethoxysilane,vinyl-tris(2-methoxyethoxy)silane and the like. Of theseorganoalkoxysilanes, gamma-methacryloxypropyltrimethoxysilane isespecially preferred.

The proportion of such crosslinking monomer employed in the process ofthe invention may range from 0.5 percent to 15 percent by weight of themonomers used in the copolymerization process. When the crosslinkingmonomer is a mixture of alkylenimine and organoalkoxysilane, the moleratio of the alkylenimine to the alpha, beta-ethylenically unsaturatedmonocarboxylic acid used to prepare the polymer is generally in therange of from 0.5:1 to 1.5:1 and the mole ratio of theorganoalkoxysilane to the alpha, beta-ethylenically unsaturatedmonocarboxylic acid used to prepare the polymer is generally in therange of from 1.5:1 to 3.5:1.

The monoethylenically-unsaturated monomer, acid monomer andcross-linking monomer are polymerized in a dispersing liquid whichsolubilizes the monomers but in which the resulting polymers areessentially not soluble and form dispersed polymer particles. Thedispersing liquid is generally a hydrocarbon medium consistingessentially of liquid aliphatic hydrocarbons. A pure aliphatichydrocarbon or a mixture of two or more may be employed. To the extentthat any particular polymer produced is mostly insoluble in thehydrocarbon medium resulting, the essentially aliphatic hydrocarbon maybe modified by the incorporation of other solvent materials such asaromatic or naphthenic hydrocarbons, and in certain instances, theamount of such non-aliphatic component may attain as high as 49 percentby weight of the entire dispersing liquid. However, the dispersingliquid preferably consists essentially of aliphatic hydrocarbons and, ingeneral, the compositions of the present invention contain less than 25percent by weight based on the weight of the dispersing liquid of anaromatic hydrocarbon and often none at all at this stage.

It is essential that the hydrocarbon be of liquid character, but it mayhave a wide boiling range from a minimum of about 30° C. (in which casehigh pressures may be needed in the polymerization) to a maximum whichmay be as high as 300° C. For most purposes, the boiling point should befrom about 50° C. up to about 235° C.

Examples of dispersing liquids useful herein are pentane, hexane,heptane, octane, mixtures of the same, and the like.

Ordinarily, the polymerizable composition of monomers and dispersingliquid should contain from about 30 to about 80 percent by weight of thedispersing liquid. It is understood, however, that the monomericsolution need contain only that amount of dispersing liquid necessary tosolubilize the monomers and maintain the resulting polymers in adispersed state after polymerization.

The monomers are polymerized in the presence of a dispersion stabilizer.The dispersion stabilizer employed in producing the microparticles ofthe invention is a compound, usually polymeric, which contains at leasttwo segments of which one segment is solvated by the dispersing liquidand a second segment is of different polarity than the first segment andis relatively insoluble (compared to the first segment) in thedispersing liquid. Although such compounds have been used in the past toprepare dispersions of polymer, in those instances it has beenconsidered necessary that the polymer produced be ungelled, film-formingand soluble in certain solvents.

Included among such dispersion stabilizers are polyacrylates andpolymethacrylates, such as poly(lauryl)methacrylate andpoly(2-ethylhexyl acrylate); diene polymers and copolymers such aspolybutadiene and degraded rubbers; aminoplast resins, particularlyhighly naphtha-tolerant compounds such as melamine-formaldehyde resinsetherified with higher alcohols (e.g., alcohols having 4 to 12 carbonatoms), for example, butanol, hexanol, 2-ethylhexanol, etc., and otheraminoplasts of similar characteristics such as certain resins based onurea, benzoguanamine, and the like; and various copolymers designed tohave the desired characteristics, for example, polyethylenevinyl acetatecopolymers.

The presently preferred dispersion stabilizers used in this inventionare graft copolymers comprising two types of polymer components of whichone segment is solvated by the aliphatic hydrocarbon solvent and isusually not associated with polymerized particles of the polymerizableethylenically unsaturated monomer and the second type is an anchorpolymer of different polarity from the first type and being relativelynon-solvatable by the aliphatic hydrocarbon solvent and capable ofanchoring with the polymerized particles of the ethylenicallyunsaturated monomer, said anchor polymer containing pendant groupscapable of copolymerizing with ethylenically unsaturated monomers.

The preferred dispersion stabilizers are comprised of two segments. Thefirst segment (A) comprises the reaction product of (1) a long-chainhydrocarbon molecule which is solvatable by the dispersing liquid andcontains a terminal reactive group and (2) an ethylenically unsaturatedcompound which is copolymerizable with the ethylenically unsaturatedmonomer to be polymerized and which contains a functional group capableof reacting with the terminal reactive groups of the long-chainhydrocarbon molecule (1).

Generally, the solvatable segment (A) is a monofunctional polymericmaterial of molecular weight of about 300 to about 3,000. These polymersmay be made, for example, by condensation reactions producing apolyester or polyether. The most convenient monomers to use are hydroxyacids or lactones which form hydroxy acid polymers. For example, ahydroxy fatty acid such as 12-hydroxystearic acid may be polymerized toform a non-polar component solvatable by such non-polar organic liquidsas aliphatic and aromatic hydrocarbons. The polyhydroxy stearic acid maythen be reacted with a compound which is copolymerizable with theacrylic monomer to be polymerized, such as glycidyl acrylate or glycidylmethacrylate. The glycidyl groups would react with the carboxyl group ofthe polyhydroxy stearic acid and the polymer segment (A) would beformed.

Somewhat more complex, but still useful, polyesters may be made byreacting diacids with diols. For example, 1,12-dodecanediol may bereacted with sebacic acid or its diacid chloride to form a componentsolvatable by aliphatic hydrocarbons.

The preferred polymeric segment (A) of the dispersion stabilizer isformed by reacting poly-(12-hydroxystearic acid) with glycidylmethacrylate.

The second polymeric segment (B) of the dispersion stabilizer is ofpolarity different from the first segment (A) and, as such, isrelatively non-solvated by the dispersing liquid and is associated withor capable of anchoring onto the acrylic polymeric particles formed bythe polymerization and contains a pendant group which is copolymerizablewith the acrylic monomer. This anchor segment (B) provides around thepolymerized particles a layer of the stabilizer. The solvated polymersegment (A) which extends outwardly from the surface of the particlesprovides a solvated barrier which sterically stabilizes the polymerizedparticles in dispersed form.

The anchor segment (B) may comprise copolymers of (1) compounds whichare readily associated with the acrylic monomer to be polymerized suchas acrylic or methacrylic esters, such as methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, butyl acrylate, butylmethacrylate, 2-ethylhexyl acrylate, octyl methacrylate, and the like,with (2) compounds which contain groups copolymerizable with the acrylicmonomer to be polymerized and which contain groups which are reactivewith the polymeric segment (A), such as glycidyl-containing acrylatesand methacrylates, such as glycidyl acrylate and glycidyl methacrylate.These copolymers are further reacted with polymerizableethylenically-unsaturated acids, such as acrylic acid, methacrylic acid,3-butenoic acid, crotonic acid, itaconic acid, and others mentionedpreviously which contain pendant groups which are copolymerizable withthe acrylic monomer.

The preferred polymeric segment (B) is a terpolymer of methylmethacrylate, glycidyl methacrylate, and methacrylic acid.

The segments (A) and (B) are usually combined entities, the segment (A)being attached to the backbone of the graft copolymer and the segment(B) being carried in or on the backbone.

The monomer solution containing the stabilizer preferably contains fromabout 1 to about 25 percent by weight of the stabilizer. That is, theamount of dispersion stabilizer used is in the range of from about 1 toabout 25 percent by weight based on the weight of monomers anddispersion stabilizer used in the copolymerization process.

The polymerization may be carried out in a conventional manner,utilizing heat and/or catalysts and varying solvents and techniques.Generally, a free radical catalyst such as cumene hydroperoxide, benzoylperoxide or similar peroxygen compound, or an azo compound such asazobis(isobutyronitrile) is employed.

The resultant non-aqueous acrylic dispersion consists essentially ofmicrogel particles (i.e., crosslinked acrylic polymer particles)dispersed therein. These particles have particle sizes ranging from 0.1to 10 microns. Depending upon the original concentration of monomersolids, non-aqueous dispersions consisting essentially of the microgelparticles can be produced by the process at relatively highconcentrations. The term "relatively high concentration" as employedherein refers to the solids level of the non-aqueus dispersion. Thus,the process of this invention permits the production of non-aqueousdispersions of microgel particles having solids contents of from 20 to60 percent by weight or even higher.

The following examples are submitted for the purpose of furtherillustrating the nature of the present invention and should not beinterpreted as a limitation on the scope thereof. All parts andpercentages in the examples as well as throughout the specification areby weight unless otherwise indicated.

EXAMPLE I

To a 5-liter flask equipped with an up-and-over condenser, agitator,thermometer, and heating mantle was charged 1900 grams of Napoleum 30 (amedium boiling naphtha from Kerr-McGee Company), 950 grams of hexane,and 950 grams of heptane. The mixture was heated to reflux (about 85°C.) and then 200 grams of methyl methacrylate, 34 grams of a dispersionstabilizer solution and 14.3 grams of azobis(isobutyronitrile) wereadded. The dispersion stabilizer solution used contained 50.3 percentsolids (viz., dispersion stabilizer) and the dispersion stabilizer was apolymer prepared by interpolymerizing 45.4 percent methyl methacrylate,4.2 percent glycidyl methacrylate, 0.9 percent methacrylic acid, and49.5 percent of a reaction product of 89.2 percentpoly-12-hydroxystearic acid and 10.8 percent glycidyl methacrylate. Thesolvent of the dispersion stabilizer solution comprised 52.1 percentbutyl acetate, 40.0 percent VM&P naphtha and 7.9 percent toluene. Afterthis addition was complete, reflux was continued for about 20 minutesand then over a 3 -hour period was added 4060 grams methyl methacrylate,226 grams of gamma-methacryloxypropyltrimethoxysilane, 595 grams of theabove dispersion stabilizer solution, 34.0 grams of methacrylic acid,34.0 grams of 2-hydroxyethyl ethylenimine, 18.0 grams ofazobis(isobutyronitrile) and 18 grams of p-octyl mercaptan. After thisaddition, reflux was continued for another 1.5 hours and the mixture wasthen cooled and filtered.

The resultant polymeric dispersion consisting essentially of crosslinkedacrylic polymer particles (i.e., microgel particles) had a total solidscontent determined at 150° C. of 54.5 percent by weight.

EXAMPLE II

To a 5-liter flask equipped with an up and over condenser, agitator,thermometer and heating mantle were charged 1250 grams of heptane, 540grams of Isopar H (a mixed aliphatic hydrocarbon having an initialboiling point of 350° F. and a dry point of 371° F. with 90 percentdistilling between 353°-357° F., available from Humble Oil and RefiningCompany), 50 grams of methyl methacrylate, 10 grams of the dispersionstabilizer solution of Example I and 4 grams ofazobis(isobutyronitrile). The mixture was heated to reflux (about 103°C.) and held for about 30 minutes. Then over a period of about 3 hourswere added 1288 grams of methyl methacrylate, 70 grams of glycidylmethacrylate, 42 grams of methacrylic acid, 4.2 grams of Armeen DMCD(dimethyl cocoamine, available from Armour Chemical Company), 200 gramsof the dispersion stabilizer solution of Example I, 14 grams of octylmercaptan and 5.6 grams of azobis(isobutyronitrile). After this additionwas completed, reflux was continued for an additional 30 minutes andthen an additional 2.8 grams of azobis(isobutyronitrile) were added.Reflux was then continued for another one hour and the mixture was thencooled and filtered.

The resultant polymeric dispersion consisting essentially of crosslinkedacrylic polymer particles (i.e., microgel particles) had a total solidscontent determined at 150° C. of 44.9 percent by weight.

According to the provisions of the Patent Statutes, there is describedabove the invention and what are now considered to be its bestembodiments. However, within the scope of the appended claims, it is tobe understood that the invention can be practiced otherwise than asspecifically described.

We claim:
 1. A dispersion containing from 20 to 60 percent by weightcrosslinked acrylic polymer microparticles having a particle size offrom 0.1 to 10 microns, said dispersion being formed by free radicaladdition copolymerization of from 0.5 to 15 percent of alpha,beta-ethylenically unsaturated monocarboxylic acid, from 70 to 66percent of at least one other copolymerizable monoethylenicallyunsaturated monomer and from 0.5 to 15 percent of crosslinking monomerselected from the group consisting of (1) epoxy group-containingcompound and (2) a mixture of alkylenimine and organoalkoxysilane,wherein:a. said epoxy group-containing compound is monoepoxide compoundwhich additionally contains ethylenic unsaturation, b. saidorganoalkoxysilane is selected from the group consisting ofacrylatoalkoxysilane, methacrylatoalkoxysilane and vinylalkoxysilane,and c. said monomer percentages are based on the weight of monomers usedin the copolymerization process,in the presence of hydrocarbondispersing liquid which is a solvent for the polymerizable monomers buta non-solvent for the resultant polymer, and polymeric dispersionstabilizer containing at least two segments of which one segment issolvated by said dispersing liquid and a second segment is of differentpolarity than said first segment and is relatively insoluble in saiddispersing liquid, wherein the reaction is carried out at elevatedtemperature such that the dispersion polymer first forms and then iscrosslinked.
 2. The dispersion of claim 1 wherein said polymericdispersion stabilizer is a graft copolymer containing two polymericsegments of which one segment is solvated by said dispersing liquid andthe second segment is an anchor polymer of different polarity to saidfirst segment and is relatively non-solvatable by said dispersing liquidand wherein said polymeric dispersion stabilizer contains pendant groupswhich have been addition copolymerized with the ethylenicallyunsaturated monomers in the copolymerization process.
 3. The dispersionof claim 2 wherein said polymeric dispersion stabilizer is formed bygraft copolymerizing the reaction product of glycidyl methacrylate andpoly-(12-hydroxystearic acid), with methyl methacrylate and glycidylmethacrylate and the resulting copolymer product containing pendantepoxy groups is reacted with methacrylic acid.
 4. The dispersion ofclaim 1 wherein said alpha, beta-ethylenically unsaturatedmonocarboxylic acid is methacrylic acid, said other copolymerizablemonoethylenically unsaturated monomer is methyl methacrylate and saidcross-linking monomer is glycidyl methacrylate.
 5. The dispersion ofclaim 1 wherein said alpha, beta-ethylenically unsaturatedmonocarboxylic acid is methacrylic acid, said other copolymerizablemonoethylenically unsaturated monomer is methyl methacrylate and saidcross-linking monomer is a mixture of hydroxyethyl ethylenimine andgamma-methacryloxypropyltrimethoxysilane.
 6. The dispersion of claim 1wherein said alpha, beta-ethylenically unsaturated monocarboxylic acidis acrylic acid or methacrylic acid.
 7. The dispersion of claim 1wherein said other copolymerizable monoethylenically unsaturated monomeris an alkyl acrylate or alkyl methacrylate.
 8. The dispersion of claim 1wherein said other copolymerizable monoethylenically unsaturated monomeris methyl methacrylate.
 9. The dispersion of claim 1 wherein saidcrosslinking monomer is a monoepoxide compound which additionallycontains ethylenic unsaturation.
 10. The dispersion of claim 9 whereinsad monoepoxide compound which additionally contains ethylenicunsaturation is glycidyl methacrylate.
 11. The dispersion of claim 1wherein said crosslinking monomer is a mixture of alkylenimine andorganoalkoxysilane selected from the group consisting ofacrylatoalkoxysilane, methacrylatoalkoxysilane and vinylalkoxysilane.12. The dispersion of claim 11 wherein said alkylenimine is hydroxyethylethylenimine and said organoalkoxysilane isgamma-methacryloxypropyltrimethoxysilane.
 13. A method of producing adispersion containing from 20 to 60 percent by weight crosslinkedacrylic polymer microparticles having a particle size of from 0.1 to 10microns, said method comprising free radical addition copolymerizingfrom 0.5 to 15 percent of alpha, beta-ethylenically unsaturatedmonocarboxylic acid, from 70 to 99 percent of at least one othercopolymerizable monoethylenically unsaturated monomer and from 0.5 to 15percent of crosslinking monomer selected from the group consisting of(1) epoxy group-containing compound and (2) a mixture of alkylenimineand organoalkoxysilane, wherein:a. said epoxy group-containing compoundis monoepoxide compound which additionally contains ethylenicunsaturation, b. said organoalkoxysilane is selected from the groupconsisting of acrylatoalkoxysilane, methacrylatoalkoxysilane andvinylalkoxysilane, and c. said monomer percentages are based on theweight of monomers used in the copolymerization process,in the presenceof hydrocarbon dispersing liquid which is a solvent for thepolymerizable monomers but a non-solvent for the resultant polymer, andpolymeric dispersion stabilizer containing at least two segments ofwhich one segment is solvated by said dispersing liquid and a secondsegment is of different polarity than said first segment and isrelatively insoluble in said dispersing liquid, wherein the reaction iscarried out at elevated temperature such that the dispersion polymerforms and then is crosslinked.
 14. The method of claim 13 wherein saidalpha, beta-ethylenically unsaturated monocarboxylic acid is acrylicacid or methacrylic acid.
 15. The method of claim 13 wherein said othercopolymerizable monoethylenically unsaturated monomer is an alkylacrylate or alkyl methacrylate.
 16. The method of claim 13 wherein saidother copolymerizable monoethylenically unsaturated monomer is methylmethacrylate.
 17. The method of claim 13 wherein said crosslinkingmonomer is a monoepoxide compound which additionally contains ethylenicunsaturation.
 18. The method of claim 17 wherein said monoepoxidecompound which additionally contains ethylenic unsaturation is glycidylmethacrylate.
 19. The method of claim 13 wherein said crosslinkingmonomer is a mixture of alkylenimine and organoalkoxysilane selectedfrom the group consisting of acrylatoalkoxysilane,methacrylatoalkoxysilane and vinylalkoxysilane.
 20. The method of claim19 wherein said alkylenimine is hydroxyethyl ethylenimine and saidorganoalkoxysilane is gamma-methacryloxypropyltrimethoxysilane.
 21. Themethod of claim 13 wherein said polymeric dispersion stabilizer is agraft copolymer containing two polymeric segments of which one segmentis solvated by said dispersing liquid and the second segment is ananchor polymer of different polarity to said first segment and isrelatively non-solvatable by said dispersing liquid and wherein saidpolymeric dispersion stabilizer contains pendant groups which areaddition copolymerized with the ethylenically unsaturated monomers inthe copolymerization process.
 22. The method of claim 21 wherein saidpolymeric dispersion stabilizer is formed by graft copolymerizing thereaction product of glycidyl methacrylate and poly-(12-hydroxystearicacid), with methyl methacrylate and glycidyl methacrylate and theresulting copolymer product containing pendant epoxy groups is reactedwith methacrylic acid.
 23. The method of claim 13 wherein said othercopolymerizable monoethylenically unsaturated monomer is methylmethacrylate, said alpha, beta-ethylenically unsaturated monocarboxylicacid is methacrylic acid, and said crosslinking monomer is a mixture ofgamma-methacryloxypropyltrimethoxysilane and hydroxyethyl ethylenimine.24. The method of claim 13 wherein said other copolymerizablemonoethylenically unsaturated monomer is methyl methacrylate, saidalpha, beta-ethylenically unsaturated monocarboxylic acid is methacrylicacid and said crosslinking monomer is glycidyl methacrylate.